Kenwood TL-911 – PA0FRI passive-grid modification – output power stops increasing above ~20 W drive
Hello,I am working on a Kenwood/Trio TL-911 amplifier with the original five 6LQ6 tubes.
I have modified the input/grid circuit following the PA0FRI TL-911 modification. The amplifier is now operating and the input matching problem I initially had has been solved, but I have one problem that I cannot explain.
The present configuration is:
- Original 5 × 6LQ6 tubes.
- RF drive is applied to G1 through the new passive input matching network.
- G2 circuit remains as in the original TL-911, except that the G2 negative voltage has been adjusted according to the modification instructions.
- G3 remains unchanged.
- Cathodes remain as in the original circuit, grounded for RF through the existing 4.7 nF capacitors.
- The original G1 capacitive trimmer has been removed.
- The original resistor associated with the G1 input coil has been removed.
- A broadband passive matching circuit has been installed between RL2-2 and G1, according to the PA0FRI modification.
- The 4:1 transformer currently installed has been checked separately with a VNA. With a 200 Ω non-inductive load on the high-impedance side, the VNA measures approximately 55 Ω and SWR 1.15 at 6.9 MHz, with good broadband behavior.
- Input SWR remains stable when RF drive is increased.
- No ALC connection is being used.
The strange behavior is the following:
On 7.1 MHz, with a 50 Ω dummy load:
20 W RF drive → approximately 140 W RF output.
Increasing the drive above approximately 20 W does not produce any significant increase in output power. The amplifier seems to enter a very clear gain compression/limiting condition.
The same behavior was previously observed with the antenna/ATU connected, so I repeated the test into a proper 50 Ω dummy load and obtained essentially the same result. Therefore the antenna and ATU can be excluded as the cause.
One additional observation may be important:
With the G2 negative voltage previously adjusted to approximately −78 V, I obtained about 200 W output with 20 W drive.
After changing it to approximately −70 V, as suggested in the modification instructions, the output became approximately 140 W with 20 W drive, but the same limiting behavior remains: increasing RF drive above 20 W does not significantly increase RF output.
At this point I would like to understand whether this behavior is caused by:
- the G1 passive-grid modification/input circuit,
- the G2 operating point,
- the TL-911 protection circuit acting prematurely,
- grid current/loading of the five 6LQ6 tubes,
- or another operating condition that must be changed after converting the input circuit.
In particular, what measurements would you recommend to determine why RF output stops increasing above approximately 20 W of drive?
I would also be interested in the expected G1/G2 voltages and currents under RF drive after this modification, and whether any other TL-911 adjustment needs to be changed from the original service-manual settings.
73 and thank you for any advice.
DL1YO